LCD polymer orienting film with a dipole moment greater than 20 debye and is bound to the substrate surface through a surface treating agent

ABSTRACT

A liquid crystal device including two substrates each having a liquid crystal orienting film formed on each opposing surface, and a liquid crystal sealed between these substrates, wherein a liquid crystal orienting film formed on the surface of at least one of the two substrates is constituted by a polyamino acid film having a rigid structure and a dipole moment that is bound to the surface of the substrate through a layer of surface treating agent.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a liquid crystal device and a method ofdriving the same.

2. Description of the Related Art

Conventionally, a liquid crystal display is popularly used, as the mostpromising compact flat panel display, for the display of a calculator ora watch, a display for an automobile, a display for a personal computer,or a moving picture display such as a compact liquid crystal television.In recent years, a liquid crystal device has been applied to not onlydisplay devices but also a shutter array, a light bulb, a switchingdevice for optical information processing, and an optical memory.Therefore, the application range of the liquid crystal device has beenwidened. As described above, in order to generally use a liquid crystaldevice in the above application fields, a higher voltage response mustbe achieved.

In order to increase the voltage response of a liquid crystal, thefollowing methods are generally used:

1) The viscosity of a liquid crystal is decreased by adding an agent tothe liquid crystal, thereby increasing the movement of liquid crystalmolecules.

2) A pre-tilt angle is increased by changing the structure of a liquidcrystal orienting film.

3) A liquid crystal other than a nematic liquid crystal, e.g., aferroelectric liquid crystal having a high sensitivity to an electricfield is used.

4) The Waveform of a driving voltage to be applied is controlled.

At present, however, a response of a liquid crystal with respect to theapplied voltage is hard to reach a response required in the aboveapplication fields. In addition, since a ferroelectric liquid crystal isnot easily synthesized, only a few ferroelectric liquid crystal devicesmanufactured as samples are reported.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a liquid crystaldevice capable of realizing a high response with respect to a voltageand a method of driving the liquid crystal device.

A liquid crystal device according to the present invention comprises twosubstrates each having a liquid crystal orienting film on each opposingsurface and a liquid crystal sealed between the substrates, wherein theliquid crystal orienting film formed on a surface of at least one of thetwo substrates is constituted by a polymer film having a rigid structureand a dipole moment that is bound to the surface of the substratethrough a layer of a surface treating agent.

In a method of driving a liquid crystal device according to the presentinvention by applying a voltage across two substrates, before aneffective driving voltage is applied, an initial voltage higher than theeffective driving voltage is applied.

Additional objects and advantages of the invention will be set forth inthe description which follows, and in part will be obvious from thedescription, or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and obtained by means ofthe instrumentalities and combinations particularly pointed out in theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate a presently preferred embodimentof the invention, and together with the general description given aboveand the detailed description of the preferred embodiment given below,serve to explain the principles of the invention.

FIG. 1 is a sectional view showing a liquid crystal cell formedaccording to Example 1 of the present invention;

FIG. 2 is a block diagram showing the circuit arrangement of a measuringapparatus used for observing the response of liquid crystal molecules;

FIG. 3 is a graph showing the response curves of the liquid crystal cellaccording to the present invention and conventional liquid crystal cellswith respect to the application and stop of a voltage;

FIG. 4 is a graph showing the response curves of the liquid crystal cellaccording to the present invention and conventional liquid crystal cellswith respect to the interval time of an application voltage;

FIG. 5 is a graph showing the response curves of the liquid crystal cellaccording to the present invention with respect to the application andstop of a voltage when a method of applying the voltage is changed;

FIG. 6 is a sectional view showing a liquid crystal cell formedaccording to Example 2 of the present invention;

FIG. 7 is a view showing the directions of the dipole moments of theliquid crystal orienting films of the liquid crystal cell in FIG. 6;

FIG. 8 is a graph showing the response curves of the liquid crystal cellaccording to Example 3 of the present invention when a method ofapplying a positive voltage is changed;

FIG. 9 is a graph showing an effect of decreasing a positive voltage inthe liquid crystal cell according to Example 3 of the present invention;

FIG. 10 is a graph showing the response curves of the liquid crystalcell according to Example 3 of the present invention when a method ofapplying a negative voltage is changed;

FIG. 11 is a graph showing an effect of decreasing a negative voltage inthe liquid crystal cell according to Example 3 of the present invention;

FIG. 12 is a graph showing an influence of the sign of a voltage appliedfirst in application of an AC voltage on the response of the liquidcrystal cell according to Example 3 of the present invention;

FIG. 13 is a graph showing the driving frequency characteristics of aresponse for a conventional liquid crystal cell having polyimide liquidcrystal orienting film; and

FIG. 14 is a graph showing the driving frequency characteristics of aresponse for a liquid crystal cell having liquid crystal orienting filmsconsisting of PBLG and PBDG according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the present invention, two substrates that have electrodes on theirsurfaces (across which a voltage can be applied) may be used assubstrates without limiting their materials. A substrate havingfunctional groups uniformly present at a high density on a surfacethereof is properly used. Of these functional groups, --OH groups aremost generally used. The --OH groups can be easily generated byacid-treating the surface of a substrate consisting of glass, sapphire,silicon, germanium, a metal, or an ITO (Indium Tin Oxide).

In the present invention, the surface of such a substrate is chemicallytreated with a surface treating agent to form a layer of surfacetreating agent, thereby increasing the functional group density on thesubstrate Surface. As the surface treating agent, silicon tetrachloride,a silane coupling agent, a titanium coupling agent, or the like is used.

The above treatment is performed as follows. A surface of substrate isacid treated to generate functional groups such as --OH groups, and thentreated with a silane coupling agent or the like. A silane couplingagent having primary amino groups, hydroxyl groups, isocyanate groups,isothiocyanate groups, or carboxyl groups is preferably used. Inaddition, a titanium coupling agent having similar functional groups mayalso be used.

In the present invention, a polymer film having a rigid structure and adipole moment is bound to the above layer of surface treating agent soas to form a liquid crystal orienting film on the substrate surface. Apolymer having functional groups which react with the functional groupsof the layer of the surface treating agent is used. The functionalgroups of the polymer may be located either at the main chain or at sidechains. Rigid portions may also be located either at the main chain orat side chains. The dipole moment value of the polymer is desirably setto be 20 debye or more because an orienting force acting on liquidcrystal molecules must be sufficiently removed by moving the polymer inthe liquid crystal orienting film when a voltage is applied. In theobtained polymer film, a rigid, rod-like polymer having a large dipolemoment is aligned such that the molecular chains of the polymermolecules are not intertwined.

An example of such a polymer--a polymer Which has a π-conjugated systemconstituted by heterocycles and a structure having anelectron-withdrawing portion and an electron-donating portion--is anoxiazine dye. Alternative examples of the above polymer includepolyamides, polyamic acids, polyesters, and polyamino acids each havingfunctional groups. Known examples of polyamino acids that form a α-helixstructure include polyalanine, polyaspartic acid, polyaspartate, apolyglutamic acid, polyglutamate, polyleucine, polylysine,polyphenylalanine, and polytyrosine. These polymers are also known asliquid crystal polymers or lyotropic liquid crystal polymers.

Polymers having a dipole moment of 350 to 4,000 debye are preferable. Inthis case, a dipole moment value falling within this range correspondsto a molecular weight of 20,000 to 250,000 in the above polyamino acid.For example, the dipole moment of poly(γ-benzyl-L-glutamate), PBLG,increases in proportion to the molecular weight, and it exceeds 2,000debye for a molecular weight of 150,000. However, when the dipole momentof the polymer becomes excessively large, electrostatic breakdown mayoccur in the liquid crystal orienting film. For this reason, asdescribed above, the dipole moment of the polymer is preferably set to4,000 debye or less.

When the polymer film consists of a polyamino acid, the polyamino acidmay consist of a mixture of an L-form and a D-form. The ratio of theL-form to the D-form is preferably set within a range of 1:3 to 3:1 as amolar ratio of repeating units. When the polyamino acid consisting of amixture of an L-form and a D-form is used, an influence of the u-helixstructure on liquid crystal alignment can be canceled. For this reason,a contrast ratio can be increased.

In the present invention, the dipole moments of polymer films having arigid structure and a dipole moment each of which constitutes a liquidcrystal orienting film formed on the surface of each of two substratesthrough a layer of surface treating agent preferably have components ofthe same direction with respect to the direction perpendicular to thesesubstrates. In the above arrangement, the voltage response of the liquidcrystal can be increased.

In addition, in the present invention, polymer films having a rigidstructure and a dipole moment each of which constitutes a liquid crystalorienting film formed on the surface of each of two substrates through alayer of a surface treating agent may have uniaxial alignment. As apractical method of causing a polymer film to have uniaxial alignment, amethod of applying a magnetic or electric field in formation of thepolymer film and a method of rubbing the polymer film after formation ofthe polymer film are known. In the present invention, if the uniaxialalignment is introduced to the polymer film serving as the liquidcrystal orienting film, the liquid crystal alignment is improved when novoltage is applied.

A liquid crystal orienting film used in the liquid crystal device of thepresent invention that is constituted by a polymer film having a rigidstructure and a dipole moment has a liquid crystal orienting forcestronger than that of a normal rubbed polyimide film when no voltage isapplied. In general, the stronger the liquid crystal orienting force ofa liquid crystal orienting film, the stronger an orienting force actingon the liquid crystal molecules becomes. For this reason, it may besupposed that the liquid crystal molecules are hard to move near theinterface with the liquid crystal orienting film, so that the responseof the liquid crystal device will be delayed, or a driving voltage willbe increased. However, when a voltage is applied to a liquid crystalorienting film used in the liquid crystal device of the presentinvention, polymer molecules in the liquid crystal orienting film aremoved due to a large dielectric anisotropy, and the alignment of thepolymer molecules is disturbed. As a result, the orienting force actingon the liquid crystal molecules is lost. For this reason, thehomeotropic alignment of liquid crystal molecules can be achieved at ahigher speed and at a lower voltage When the above liquid crystalorienting film is used than when a normal rubbed polyimide film is used.When the liquid crystal orienting film is used, the orienting forceacting on liquid crystal molecules is not lost even when a voltage isapplied.

In the liquid crystal orienting film described above, the molecularchains of adjacent polymer molecules are not intertwined. For thisreason, when voltage application is stopped, the alignment of thepolymer molecules is rapidly restored due to a hysteresis effect of thealignment. Therefore, the strong liquid crystal orienting force of thepolymer film is restored, and the alignment of the liquid crystalmolecules to which no voltage is applied can be reproduced at a highspeed.

When the liquid crystal device having the above arrangement is to bedriven, if an initial voltage higher than an effective driving voltagebefore the effective driving voltage is applied, an orienting forceacting on the liquid crystal molecules by the liquid crystal orientingfilm is lost during application of the high initial voltage. For thisreason, a higher response can be realized.

In this case, the application time of the initial voltage is preferablyset to be 40 μs or more. In this manner, when the initial voltage higherthan the effective driving voltage is applied to the liquid crystalorienting film first, alignment of the liquid crystal orienting filmitself is rapidly disturbed, and an orienting force acting on the liquidcrystal of a bulk apart from the liquid crystal orienting film is alsolost. For this reason, even when a lower effective driving voltage isused (lower than a constant driving voltage which is regularly used),the liquid crystal can be driven. In addition, as described above, sincethe application time of the high initial voltage applied is firstsatisfactorily set to be about 40 μs, the effective driving voltage canbe set to be 0.8 or more times and less than a regular driving voltageto reduce power consumption.

A preferred method of driving the aforementioned liquid crystal devicewill be described below. When this liquid crystal device is to bedriven, it is preferable to apply an initial voltage higher than theeffective driving voltage before the effective driving voltage isapplied. It is also preferable to apply a positive voltage to thesubstrate on which the polymer film has the dipole moment directed froma substrate side to the film surface. Similarly, it is preferable toapply a negative voltage to the substrate on which the polymer film hasthe dipole moment directed from a film surface side to the substrate.

In addition, in a method of applying an AC voltage to the liquid crystaldevice of the present invention, a voltage applied first is desirablychanged in accordance with the direction of the dipole moment of apolymer film serving as a liquid crystal orienting film. Morespecifically, when the dipole moment is directed from the substrate sideto the film surface, an AC voltage is desirably applied to the liquidcrystal device such that the first pulse applied to the substrate havingthis polymer film formed thereon has a negative voltage. Likewise, whenthe dipole moment of the polymer film is directed from the film surfaceside to the substrate, an AC voltage is desirably applied to the liquidcrystal device such that the first pulse applied to the substrate havingthe polymer film formed thereon has a positive voltage. For example,when PBLG is used as a polymer constituting the liquid crystal orientingfilm, the carboxyl group is bound to the substrate and the amino groupis present on the film surface side. In this case, when a negativevoltage is first applied to this substrate, the liquid crystal devicecan be driven at a higher speed.

EXAMPLES

Examples of the present invention will be described below With referenceto the accompanying drawings.

Example 1

A liquid crystal cell according to this example shown in FIG. 1 isformed as follows. A glass substrate 1 (20 mm×5 mm) having an ITOelectrode 2 formed on a surface thereof is prepared. This glasssubstrate 1 is washed with flowing distilled water for an hour, washedwith methylene chloride by ultrasonic cleaning for 5 minutes, and thenwashed with a flon vapor. 1.25 g ofN-(2-aminoethyl)-3-aminopropylmethyldimethoxysilane (silane couplingagent available from Toshiba Silicone, tradename: TSL-8345) serving as asurface treating agent are mixed with 25 cc of distilled water and 1 ccof an acetic acid. This mixture is stirred, thereby preparing a solutionof Ph 4 for surface treatment. The glass substrate 1 is dipped into thissolution, and kept in the solution at room temperature for 8 hours. Theglass substrate 1 is washed with methylene chloride, distilled water,and acetone in this order and then dried. As a result, a layer ofsurface treating agent 3 terminated with amino groups is formed on thesurface of the ITO electrode 2.

Subsequently, 3 g of poly(γ-benzyl-L-glutamate) (PBLG, available fromSigma, molecular weight: 116,000), a polymer having a rigid structureand a dipole moment of 1,000 debye or more, are dissolved in 50 cc ofdry methylene chloride. 1.4 g of DCC (dicyclohexylcarbodiimide) are thenadded to the mixture which is reacted at 0° C. for 20 minutes. The glasssubstrate 1 is dipped into this solution, and the temperature of thesolution is slowly raised to room temperature. In this state, the glasssubstrate 1 is reacted with the solution overnight. Upon completion ofthe reaction, the glass substrate 1 is washed With methylene chloride,acetone, distilled water, and acetone in this order to removeby-products and excessive PBLG from the glass substrate 1. This forms aliquid crystal orienting film 4 constituted by a PBLG monolayer film onthe layer of the surface treatment agent 3.

The thickness of the PBLG monolayer film formed as described above isestimated to be about 75 nm assuming that all the molecules of the filmare aligned perpendicularly to the substrate surface. The contact angleof water measured before the formation of the PBLG monolayer film is22°, but that measured after the formation of the PBLG monolayer film is51°. For this reason, it is found that the PBLG monolayer film ishydrophobic due to the PBLG skeleton. In addition, when an atomic ratioon the surface of the PBLG monolayer film is measured by an XPS method,the ratio of carbon to nitrogen is about 12:1.It is confirmed that theratio coincides with that of the composition of PBLG.

As shown in FIG. 1, a substrate is obtained by the above method suchthat the liquid crystal orienting film 4 consisting of PBLG is bound tothe surface of the ITO electrode 2 on the glass substrate 1 through thelayer of the surface treating agent 3. This substrate is used as,thefirst substrate. The substrate obtained such that a polyimide liquidcrystal orienting film 7 to which a rubbing process is not performed isformed on the surface of a glass substrate 5 having an ITO electrode 6is used as the second substrate. The first substrate is set to opposethe second substrate through spacers 8, and they are sealed to give acell gap of about 10 μm. Thereafter, pentylcyanobiphenyl (5CB, availablefrom BHD) in the form of liquid crystal molecules are injected into thecell, sealed, heated, and then gradually cooled to align a liquidcrystal 9. A power supply 10 is connected to the ITO electrodes 2 and 6,so that a predetermined voltage is applied across these ITO electrodes.

Comparative Example 1

A liquid crystal cell is formed in the same manner as described inExample 1 except that each of the substrates used are formed withoutperforming a rubbing process on the liquid crystal orienting film.

Comparative Example 2

A liquid crystal cell is formed in the same manner as described inExample 1 except that each of the substrates used are formed afterperforming a rubbing process on the liquid crystal orienting film.

In each of the above three types of liquid crystal cells (Example 1,Comparative Example 1, and Comparative Example 2), the response ofliquid crystal molecules with respect to an application of a pulsedelectric field is examined using a measuring apparatus shown in FIG. 2.In this experiment, infrared absorption of 2,225 cm⁻¹ attributed to thestretching vibration of the cyano group of 5CB is time-resolved andmeasured, thereby observing the motion of the liquid crystal molecules.Since the motion of the cyano groups corresponds to the motion of aliquid crystal director, the motion of the liquid crystal director thatdetermines the optical response of the liquid crystal cell can beobserved by observing the motion of the cyano groups.

Referring to FIG. 2, a pulse signal generated by a function generator 21is amplified by a high-speed amplifier 22 and then applied to a liquidcrystal cell 100. On the other hand, infrared rays from a light source23 are radiated on the liquid crystal cell 100 through a polarizer 24.The infrared rays of 2,225 cm⁻¹ from the liquid crystal cell 100 arespectrally analyzed by a dispersive-type infrared spectrometer 25 anddetected by a highly sensitive mercury-cadmium-tellurium (MET) infrareddetector 26. A time resolution is 500 ns, and a detection sensitivityconverted into a director angle is about 0.01 degree. In this case, whenthe alignment of the liquid crystal molecules changes by applying anelectric field to the cell, an absorbance changes. This change isamplified by an AC amplifier 27, amplified by a main amplifier 28, andthen input to a digital sampling oscilloscope 29 and, at the same time,integrated by a box-car integrator 30. The measuring system is entirelycontrolled by a computer 31.

FIG. 3 shows a change in infrared absorption with time where a pulsedelectric field is applied to a liquid crystal cell under the conditions:a voltage of 10 V, a pulse width of 2 ms, and a frequency of 5 Hz(interval time: 198 ms). A time resolution is 10 μs. In this case,although the direction of the director of the liquid crystal moleculeshas a certain tilt angle with respect to the cell surface in an initialstate, the direction is almost parallel to the cell surface. Thedirection is rotated with voltage application and becomes almostperpendicular to the cell surface. The direction of the cyano groupschanges like the direction of the liquid crystal molecules, and themagnitude of infrared absorption decreases with voltage application.When the voltage is removed, the direction of the liquid crystalmolecules is returned to the initial direction, and the magnitude of theinfrared absorption is returned to the original magnitude.

As is apparent from FIG. 3, the change in infrared absorption when theelectric field application is ended (2 ms after the field application)is largest in the liquid crystal cell of Example 1, and is 10 or moretimes that of the liquid crystal cell of Comparative Example 1 or 2. Itis found that a response is considerably increased in the liquid crystalcell of Example 1. Therefore, it is expected that when driven at thesame voltage, the liquid crystal cell of Example 1, in which a PBLG filmis used as a liquid crystal orienting film, can obtain a contrast ratiohigher than that of the liquid crystal cell of Comparative Example 1 or2 in which a polyimide film is used as a liquid crystal orienting film.The liquid crystal cell of Example 1, however, can obtain the samecontrast ratio as in Comparative Example 1 or 2 at a voltage lower thanthat applied to the liquid crystal cell of Comparative Example 1 or 2.

In each of the liquid crystal cells of Example 1 and ComparativeExamples 1 and 2, the frequency of an applied voltage, i.e., an intervaltime, is changed to measure the time required for returning the state ofthe liquid crystal molecules to the initial state. When the frequency ishigh, after an electric field is removed, a next pulsed electric fieldis applied before the state of the liquid crystal molecules iscompletely returned to the initial state. For this reason, hysteresis ofelectric field application is left in the movement of the liquid crystalmolecules. However, when the frequency is lower than a certainfrequency, i.e., when the interval time is longer than a certain time,the next pulsed electric field is applied in a state wherein the liquidcrystal molecules is completely returned to the initial state. For thisreason, the motion of the liquid crystal molecules becomes constant.While the frequency of a pulsed electric field is varied, a timerequired for causing the liquid crystal molecules to rise (time at which50% of the maximum change in infrared absorption achieved by pulsedelectric field application is exhibited) is measured, and an intervaltime required for setting the value to be constant is examined. Theresults are shown in FIG. 4.

As is apparent from FIG. 4, although the liquid crystal cell of Example1 is returned to the initial state within about 400 ms, that ofcomparative Example 2 requires a time of about 800 ms, and that ofcomparative Example 1 requires a time of 1,600 ms or more. As isapparent from these results, since the alignment of the PBLG filmconstituting the liquid crystal orienting film of the liquid crystalcell of Example 1 is rapidly restored to recover a strong orientingforce acting on the liquid crystal molecules, the state of the liquidcrystal molecules can be returned to the initial state within a timeshorter than that of a liquid crystal orienting film consisting of arubbed polyimide film. The time obtained from this experiment is longerthan a rise time obtained by optical measurement. This means that arelatively long time is required for completely returning the state ofnot only the liquid crystal molecules of a bulk but also the liquidcrystal molecules near the liquid crystal orienting film to the initialstate.

In order to realize a higher response, a method of applying an electricfield is examined. In this case, the same experiment as described aboveis performed to the liquid crystal cell of Example 1 using two methods,i.e., a method (A) in which a voltage of 10 V is continuously appliedfor me and a method (B) in which, after an initial voltage of 15 V isapplied for 0.5 ms, a voltage of 10 V is applied for 0.5 ms. Resultsobtained by these experiments are shown in FIG. S.

As is apparent from FIG. 5, after a voltage is applied, the timerequired for causing a change to reach 50% of the maximum change isabout 0.6 ms in method A and about 0.3 ms in method B. The valueobtained in method B is about half of that obtained in method A. In theliquid crystal cell of Example 1, it is confirmed that the liquidcrystal molecules can be caused to rise at a high speed by applying aninitial voltage higher than an effective driving voltage to the liquidcrystal cell before the effective driving voltage is applied.

Example 2 Example 2a

A liquid crystal cell according to this example shown in FIG. 6 isformed as follows. A first glass substrate 1 (20 mm×5 mm) having an ITOfilm 2 formed on a surface thereof is washed with flowing distilledwater for an hour, washed with methylene chloride by ultrasonic cleaningfor 5 minutes, and then washed with a flon vapor. 1.25 g ofN-(2-aminoethyl)-3-aminopropylmethyldimethoxy-silane (silane couplingagent available from Toshiba Silicone, tradename: TSL-8345) serving as asurface treating agent are mixed with 25 cc of distilled B water and 1cc or an acetic acid. This mixture is stirred, thereby preparing asolution of pH 4 for surface treatment. The first glass substrate 1 isdipped into this solution, and kept in the solution at room temperaturefor 8 hours. The first glass substrate 1 is washed with methylenechloride, distilled water, and acetone in this order and then dried. Asa result, a layer of the surface treating agent 3 terminated with aminogroups is formed on the substrate surface.

A second glass substrate 11 (20 mm×5 mm) having an ITO film 12 formed ona surface thereof is washed with flowing distilled water for an hour,washed with methylene chloride by ultrasonic cleaning for 5 minutes, andthen washed with a flon vapor. 1.25 g of a silane coupling agent havingisocyanate groups (available from Chisso) serving as a surface treatingagent are mixed with 25 cc of distilled water and 1 cc of an aceticacid. This mixture is stirred, thereby preparing a solution of pH 4 forsurface treatment. The second glass substrate 11 is dipped into thissolution, and kept in the solution at room temperature for 8 hours. Thesecond glass substrate 11 is washed with methylene chloride, distilledwater, and acetone in this order and then dried. As a result, a layer ofsurface treating agent 13 terminated with isocyanate groups is formed onthe substrate surface.

Subsequently, 3 g of poly(γ-benzyl-L-glutamate (PBLG, available fromSigma, molecular weight: 116,000) are dissolved in 50 cc of drymethylene chloride. 1.4 g of DCC (dicyclohexylcarbodiimide) are thenadded and the mixture is reacted at 0° C. for 20 minutes. The first andsecond glass substrates 1 and 11 are dipped into this solution, and thetemperature of the solution is slowly raised to room temperature. Inthis state, the first and second glass substrates 1 and 11 are reactedwith the solution overnight. Upon completion of the reaction, the firstand second glass substrates 1 and 11 are washed with methylene chloride,acetone, distilled water, and acetone in this order to removeby-products and excessive PBLG from the first and second 91asssubstrates 1 and 11. The result is the formation of liquid crystalorienting films 4 and 14 each constituted by a PBLG monolayer film onthe layers of surface treating agents 3 and 13, respectively.

In the first glass substrate 1, the PBLG monolayer film is bound to thesubstrate 1 such that a carboxyl group at one end of each PBLG moleculereacts with one of the amino groups on the substrate surface side, andamino groups are present on the film surface side. Therefore, the dipolemoment of S this PBLG monolayer film is directed from the substrate sideto the film surface side. On the other hand, in the second glasssubstrate 11, the PBLG monolayer film is bound to the substrate suchthat an amino group at one end of each PBLG molecule reacts with one ofthe isocyanate groups on the surface of the substrate, and carboxylgroups are present on the film surface side. Therefore, the dipolemoment of this PBLG monolayer film is directed from the film surfaceside to the substrate side.

The thickness of each of the PBLG monolayer films formed as describedabove is estimated to be about 75 nm, assuming that all the molecules ofthe film are aligned perpendicularly to the substrate surface. Thecontact angle of water measured before the formation of the PBLGmonolayer film is 22°; the angle measured after the formation of thePBLG monolayer film is 51°. For this reason, it is found that the PBLGmonolayer film is hydrophobic due to the PBLG skeleton. In addition,when an atomic ratio on the surface of the PBLG monolayer film ismeasured by an XPS method, the ratio of carbon to nitrogen is about12:1.This ratio coincides with that of the composition of PBLG.

As shown in FIG. 6, the first and second glass substrates 1 and 11 areset to oppose each other through spacers 8 and sealed to give a cell gapof about 10 pm. Thereafter, pentylcyanobiphenyl (5CB, available fromBHD) in the form of liquid crystal molecules are injected into the cell,sealed, heated, and then gradually cooled to align a liquid crystal 9.FIG. 7 diagrams the directions of the dipole moments of the PBLG films.The dipole moments of the PBLG films constituting the liquid crystalorienting films 4 and 14 respectively formed on the two glass substrates1 and 11 through the layers of the surface treating agents 3 and 13 havethe components of the same direction when viewed in the directionperpendicular to the substrates.

Example 2b

Liquid crystal orienting films are formed on the first and second glasssubstrates in the same manner as described above except that PBLG andpoly(γ-benzyl-L-glutamate) (PBDG, available from Sigma, molecularweight: 298,000) mixed with each other at a molar ratio of 1:1 asrepeating units are used in place of poly(γ-benzyl-L-glutamate) (psLGtavailable from Sigma, molecular weight: 116,000). Note that PBDG, likePBLG, has a dipole moment of 1,000 debye or more. A liquid crystal cellsimilar to that shown in FIG. 6 is formed using these glass substrates.

Example 2c

A rubbing process is performed to liquid crystal orienting films on thesurfaces of first and second glass substrates formed as in Example 2a. Aliquid crystal cell as in FIG. 6 is formed using these glass substrates.

Example 2d

A rubbing process is performed to liquid crystal orienting films on thesurfaces of first and second glass substrates formed as in Example 2b. Aliquid crystal cell as in FIG. 6 is formed using these glass substrates.

In each of the liquid crystal cells of Examples 2a to 2d, since theliquid crystal orienting films respectively formed on the surfaces ofthe two substrates are moved in the same direction by applying avoltage, the voltage applied can be reduced. In each of the liquidcrystal cells, a threshold voltage obtained by the time-resolvingmeasurement described above is 2.2 V. In addition, a constant drivingvoltage is applied to each of the liquid crystal cells at a frequency of30 Hz (application time per pulse is 33 ms) to examine itstransmittance, and the relationship between the voltage and thetransmittance is examined. As a result, it is found that each of theliquid crystal cells can perform a display operation at ±3 v. Thisvoltage is hereinafter referred to as a regular driving voltagehereinafter.

Note that the liquid crystal cells of Comparative Examples 1 and 2 eachhaving the above polyimide liquid crystal orienting film compared withthe liquid crystal cells of Examples 2a to 2d.

In each of the liquid crystal cells of Comparative Examples 1 and 2, adichroic ratio of long axis direction to short axis direction (when noelectric field is applied) is calculated by the stretching vibration ofa cyano group of 2,225 cm⁻¹ obtained by infrared absorptionspectroscopy. Response times to an electric field (rise time τ_(r) anddecay time τ_(d)) are measured when each of the liquid crystal cells ofExamples 2a to 2d is driven such that a rectangular voltage of ±3 v(regular driving voltage) having a ratio of 1:1 is applied to the liquidcrystal cell at a frequency of 30 Hz (pulse width: 33 ms). The rise timeτ_(r) is the time required for decreasing a brightness to 10% after thevoltage is turned on, and the decay time τ_(d) is the time required forrecovering a brightness to 90% after the voltage is turned off. Each ofthe liquid crystal cells of Comparative Examples 1 and 2 is driven at avoltage of ±7 V, thereby performing the same experiment as describedabove. In addition, a physical contrast ratio CR=I^(OFF) :I^(ON) ismeasured using an He-Ne laser as a light source. The results obtained bythese measurements are described in Table 1.

                  TABLE 1                                                         ______________________________________                                                   Dichroic                                                                             τ.sub.r                                                                            τ.sub.d                                                   Ratio  (ms)     (ms)   CR                                          ______________________________________                                        Comparative  1        8        100   5:1                                      Example 1                                                                     Comparative  3        10       20   150:1                                     Example 2                                                                     Example 2a   1.2      8        20    75:1                                     Example 2b   1.4      8        19   120:1                                     Example 2c   2.5      6.5      16   170:1                                     Example 2d   3        5        15   200:1                                     ______________________________________                                    

As is apparent from Table 1, each of the liquid crystal cells ofExamples 2a to 2d has a short rise time τ_(r), a short decay time τ_(d),and a high physical contrast ratio CR.

Example 3

A liquid crystal cell as in Example 2d and shown in FIG. 6 is formed asfollows. A first glass substrate (20 mm×5 mm) 1 having an ITO electrode2 formed on a surface thereof is prepared, washed with flowing distilledwater for an hour, washed with methylene chloride by ultrasonic cleaningfor 5 minutes, and then washed with a flon vapor. 1.25 g ofN-(2-aminoethyl)-3-aminopropylmethyldimethoxy-silane (silane couplingagent available from Toshiba silicone, tradename: TSL-8345) serving as asurface treating agent are mixed with 25 cc of distilled water and 1 ccof an acetic acid. This mixture is stirred, thereby preparing a solutionof Ph 4 for surface treatment. The first glass substrate 1 is dippedinto this solution, and kept in the solution at room temperature for 8hours. The first glass substrate 1 is washed with methylene chloride,distilled water, and acetone in this order and then dried. As a result,a layer of the surface treating agent 3 terminated with amino groups isformed on the substrate surface.

A second glass substrate 11 (20 mm×5 mm) having an ITO film 12 formed ona surface thereof is washed with flowing distilled water for an hour,washed with methylene chloride by ultrasonic cleaning for 5 minutes, andthen washed with a flon vapor. 1.25 g of a silane coupling agent havingisocyanate groups (available from Chisso) serving as a surface treatingagent are mixed with 25 cc of distilled water and 1 cc of an aceticacid. This mixture is stirred, thereby preparing a solution of pH 4 forsurface treatment. The second glass substrate 11 is dipped into thissolution, and kept in the solution at room temperature for 8 hours. Thesecond glass substrate 11 is washed with methylene chloride, distilledwater, and acetone in this order and then dried. As a result, a layer ofsurface treating agent 13 terminated with isocyanate groups is formed onthe substrate surface.

Subsequently, as in Example 2, 3 g of a mixture obtained by mixingpoly(γ-benzyl-L-glutamate) (PBLG, available from Sigma, molecularweight: 116,000) and poly(γ-benzyl-D-glutamate) (PBDG, available fromSigma, molecular weight: 298,000) with each other at a ratio of 1:1 aredissolved in 50 cc of dry methylene chloride. 1.4 g of DCC(dicyclohexylcarbodiimide) are then added to the mixture and reacted at0° C. for 20 minutes. The first and second glass substrates 1 and 11 aredipped into this solution, and the temperature of the solution is slowlyraised to room temperature. In this state, the first and second glasssubstrates 1 and 11 are reacted with the solution overnight. Uponcompletion of the reaction, the first and second glass substrates 1 and11 are washed with methylene chloride, acetone, distilled water, andacetone in this order to remove by-products and excessive PBLG and PBDGfrom the first and second glass substrates 1 and 11. This results in theformation of liquid Crystal orienting films 4 and 14 constituted bymonolayer films containing PBLG and PBDG on the layers of surfacetreating agents 3 and 13, respectively. A rubbing process is performedto the liquid crystal orienting films 4 and 14. Thereafter, a liquidcrystal cell having a structure shown in FIG. 6 is formed.

In a liquid crystal cell according to the present invention, asdescribed in relation to FIG. 5, it is expected that a voltage responsecan be increased by designing a method of removing a liquid crystalorienting force generated by a liquid crystal orienting film. Therefore,the dynamic characteristics of the liquid crystal cell of this exampleare measured using the same measuring apparatus as shown in FIG. 2, andconditions for a driving method are examined in detail.

The threshold voltage of this liquid crystal cell is 2.2 V as describedabove. When a voltage of 2.4 V is applied to the liquid crystal cell,the liquid crystal molecules begin to move about 1 ms after the voltageapplication.

As in the experiment shown in FIG. 5, it is expected that a liquidcrystal orienting force generated by the liquid crystal orienting filmcan be quickly lost by applying a high initial voltage to the liquidcrystal cell first, and the following experiment is performed.

FIG. 8 shows results obtained when a voltage of 2.2 V equal to athreshold voltage is applied to the liquid crystal cell after a voltageof 4.4 V (twice the threshold voltage) is applied. The total time forapplying the voltage is set to 2 ms within a range in which liquidcrystal molecules can be satisfactorily observed. This unit time of 2 msis divided into 32. The application time of the initial voltage of 4.4 Vis changed in an integer multiple of 2 ms×1/32. Referring to FIG. 8, theconditions for the application time of the initial voltage of 4.4 v aresimply expressed by x/32 (X:O to 32).

As is apparent from FIG. 8, although the liquid crystal does not move atall when the voltage of 4.4 V is not applied to the liquid crystal cell,the liquid crystal moves slightly when the voltage of 4.4 v is appliedfor 2 ms×1/32 (62.5 μs), and the liquid crystal moves markedly when thevoltage is applied for 2 ms×8/32 (0.5 ms) or more.

In addition, it is expected that, When a high initial voltage is appliedfirst, a voltage applied subsequent to the initial voltage may bedecreased to a lower voltage than the threshold voltage, so that thefollowing experiment is performed. FIG. 9 shows results obtained when avoltage of n v (2.2 V, 1.8 V, or 1.5 V) was applied for a time of 2ms×31/32 after an initial voltage of 4.4 V was applied for 2 ms×1/32(62.5 μs). As is apparent from FIG. 9, if the initial voltage of 4.4 vis applied to the liquid crystal cell first, even when an effectivedriving voltage subsequently applied to the initial voltage is decreasedto 1.8 V (80% of the threshold voltage), the liquid crystal can stillmove.

FIGS. 10 and 11 show the results corresponding to FIGS. 8 and 9 where anegative voltage was applied to the first substrate of the liquidcrystal cell of this example.

In this case, the threshold voltage obtained when a negative voltage wasapplied to the first substrate was -6 V. The absolute value of thethreshold voltage obtained when the negative voltage was applied to thefirst substrate as described above was greater than that obtained when apositive voltage was applied the first substrate. This is because whenPBLG and PBDG constitute the liquid crystal orienting film of the firstsubstrate, carboxyl groups are bound to the substrate, amino groups arepresent on the film surface side, and a dipole moment is directed fromthe substrate side to the film surface side. When PBLG and PBDGconstitute the liquid crystal orienting film of the second substrate,amino groups are bound to the substrate, carboxyl groups are present atthe ends on the film surface side, and a dipole moment is directed fromthe film surface side to the substrate side.

When a voltage was applied to the liquid crystal cell such that thefirst and second substrates respectively were positive and negative, thepolyamino acid was rotated in a direction perpendicular to thesubstrates. This is because a repulsive force acts on the dipole momentof the liquid crystal orienting film of each of the first and secondsubstrates. However, when a voltage was applied to the liquid crystalcell such that the first and second substrates respectively werenegative and positive, the polyamino acid was rotated in a directionparallel to the substrates. This is because an attractive force acts onthe dipole moment of the liquid crystal orienting film of each of thefirst and second substrates. In this case, since the energy of theformer is lower than that of the latter, the absolute value of thethreshold voltage obtained when a voltage is applied to the liquidcrystal cell to make the first substrate positive decreases.

As is apparent from FIG. 10, When a pulse voltage of -12 V (twice thethreshold voltage) was applied to the liquid crystal cell for a time of2 ms×1/32 (62.5 μs), the liquid crystal was able to move. As is apparentfrom FIG. 11, when an initial voltage of -12 v was applied to the liquidcrystal cell for 2 ms×1/32 (62.5 μs), even when a voltage subsequentlyapplied to the initial voltage was decreased to -4.5 V (75% of thethreshold voltage of -6 V), the liquid crystal was still able to move.

On the other hand, in Comparative Example 1 or 2, where a polyimide filmwas used as a liquid crystal orienting film, even when a high initialvoltage was applied first, when a voltage subsequently applied to theinitial voltage was decreased to a lower voltage than the thresholdvoltage, the movement of the liquid crystal cannot be recognized. Forthis reason, the voltage cannot be decreased.

As described above, if a liquid crystal orienting film used in theliquid crystal cell of the present invention loses an orienting forceacting on a liquid crystal, the orienting force rarely acts on theliquid crystal molecules of a bulk apart from the liquid crystalorienting film. For this reason, the voltage response of the liquidcrystal observed in an extremely short time period from the start of theabove voltage application is expected to be applied to a voltage rangein which a display operation can be performed. A resulting high responseof display operation and decrease in effective driving voltage can beexpected. A driving method used when a display operation was performedusing the liquid crystal cell of this example is now examined.

An experiment was performed in which, after an initial voltage higherthan a regular driving voltage (3 V) was applied to the liquid crystalcell for a period of time a half the unit time of 2 ms, an effectivedriving voltage (2.4 V) was 0.8 times the regular driving voltageapplied for the remaining period of time. In this case, a voltage of 0.8times the regular driving voltage was used because the minimum voltagerequired for moving liquid crystal molecules is set to 80% or more ofthe threshold voltage in the above experiment (where the liquid crystalis moved for the extremely short time from the start of voltageapplication). The initial voltage first applied was gradually decreasedfrom a voltage of twice the regular driving voltage. It was thendetermined how many times the minimum initial voltage at which a displayoperation can be performed corresponds to the regular driving voltage.It was found that the minimum initial voltage was 3.5 V (1.16 times theregular driving voltage).

After an initial voltage (6 V) of twice the regular driving voltage wasapplied to the liquid crystal cell for a predetermined period of time,an effective driving voltage (2.4 V) of 0.8 times the regular drivingvoltage was applied for the remaining period of time. The applicationtime of the initial voltage was gradually decreased from one half theunit time of 2 ms, and the shortest application time of the initialvoltage for lo performing a display operation was examined. In thiscase, if the application time of the initial voltage is decreased byabout 1/10 or less of the unit time, power consumption is lower thanwhen only a constant regular driving voltage is applied. It was foundthat the application time of the initial voltage is suitably set to 1/50the unit time of 2 ms.

To summarize, after an initial voltage (in this case, 6 V) of twice theregular driving voltage is applied to the liquid crystal cell for 1/50the unit time of 2 ms, i.e., about 40 μs, and then an effective drivingvoltage of 0.8 times the regular driving voltage is applied, a displayoperation can be satisfactorily performed. Power consumption can bereduced by about 17.5% with respect to that used when only a constantvoltage is applied for 2 ms.

The same experiment described above was performed to the same liquidcrystal cell described in Example 2b. Here, non-rubbed films of PBLG andPBDG were used as the liquid crystal orienting films of the first andsecond substrates.

An experiment was performed in which, after an initial voltage higherthan the regular driving voltage (3 V) was applied to the liquid crystalcell for one half the unit time of 2 ms, an effective driving voltage(2.4 V) of 0.8 times the regular driving voltage was applied for theremaining period of time. The initial voltage first applied wasgradually decreased from a voltage of twice the regular driving voltage.It was examined how many times the minimum initial voltage at which adisplay operation can be performed corresponds to the regular drivingvoltage. It was found that the minimum initial voltage was 4 V (1.33times the regular driving voltage).

After an initial voltage (6 V) of twice the regular driving voltage wasapplied to the liquid crystal cell for a predetermined period of time,an effective driving voltage (2.4 V) of 0.8 times the regular drivingvoltage was applied for the remaining period of time. The applicationtime of the initial voltage was gradually decreased from one half theunit time of 2 ms. The shortest application time of the initial voltagefor performing a display operation was then examined. It was found thatthe application time of the initial voltage can suitably be set to 1/50the unit time of 2 ms.

To summarize, after an initial voltage (6 V) of twice the regulardriving voltage is applied to the liquid crystal cell for 1/50 the unittime of 2 ms, i.e., about 40 μs, and then an effective driving voltageof 0.8 times the regular driving voltage is applied, a display operationcan be satisfactorily performed. Power consumption can be reduced byabout 17.5% with respect to that used when only a constant voltage isapplied for 2 ms.

AC voltages were applied to the liquid crystal cell of Example 3 for 2ms to examine the influence of the sign of the first pulse of the ACvoltage used. The results are shown in FIG. 12 The pulse width was setto be 2 ms×1/32 (62.5 μs). An AC voltage was applied to the liquidcrystal cell as is apparent from FIG. 12, when the first pulse appliedto a substrate on which a polymer film having a dipole moment directedfrom the film surface side to the substrate is formed has a positivevoltage, and the first pulse applied to a substrate on which a polymerfilm having a dipole moment directed from the substrate side to the filmsurface is formed has a negative voltage, the voltage response in liquidcrystal driving can be increased.

The preferable voltage polarity of the first pulse used when such an ACvoltage is applied is opposite to the preferable voltage polarity usedwhen the initial voltage and effective driving voltage have thewaveforms shown in FIGS. 8 and 9,

FIGS. 13 and 14 show results obtained by examining the frequency of adriving voltage for a liquid crystal cell in which each liquid crystalorienting film consists of polyimide (FIG. 13) or PBLG and PBDG (FIG.14). The abscissa indicates frequency, and the ordinate indicates themagnitude of infrared absorption corresponding to the movement of aliquid crystal. As shown in FIG. 13, when the liquid crystal orientingfilm consists of polyimide, the liquid crystal rarely moves at afrequency of about 30 kHz. On the other hand, as shown in FIG. 14, whenthe liquid crystal orienting films consist of PBLG and PBDG, after themovement of the liquid crystal initially decreases, the movement of theliquid crystal recovers, and the movement of the liquid crystal can beobserved at a frequency of 1 MHz or more. These results demonstrate thatthe liquid crystal cell of the present invention can be driven in ahigh-frequency region.

Additional advantages and modifications of this invention will readilyoccur to those skilled in the art. Therefore, the invention in itsbroader aspects is not limited to the specific details, representativedevices, and illustrated examples shown and described herein.Accordingly, various modifications may be made without departing fromthe spirit or scope of the general inventive concept as defined by theappended claims and their equivalents.

What is claimed is:
 1. A liquid crystal device comprising:twosubstrates, each substrate having an inner and an outer surface, thesubstrates positioned opposite and substantially parallel to one anothersuch that the inner surfaces are in closer proximity than the outersurfaces; a surface treating agent layer, the layer formed by chemicallytreating the inner surface of each respective substrate; a liquidcrystal orienting film bound to the inner surface of each substratethrough the surface treating agent layer, the liquid crystal orientingfilm of at least one of the two substrates constituted by a polymer, thepolymer having a dipole moment of not less than 20 debye; and a liquidcrystal sealed between the substrates, such that when a voltage isapplied to the liquid crystal orienting film, alignment of molecules inthe liquid crystal orienting film is disturbed.
 2. The device accordingto claim 1, wherein the polymer has a dipole moment of not less than 350debye.
 3. The device according to claim 1, wherein the polymer consistsessentially of a polyamino acid.
 4. The device according to claim 3,wherein the polyamino acid has a molecular weight of not less than20,000.
 5. The device according to claim 3, wherein the polyamino acidconsists of a mixture of an L-form and a D-form.
 6. The device accordingto claim 5, wherein a ratio of the L-form to the D-form falls within arange of 1:3 to 3:1 as a molar ratio of repeating units.
 7. A liquidcrystal device comprising:two substrates, each substrate having an innerand an outer surface, the substrates positioned opposite andsubstantially parallel to one another such that the inner surfaces arein closer proximity than the outer surfaces; a surface treating agentlayer, the layer formed by chemically treating the inner surface of eachrespective substrate; a liquid crystal orienting film bound to the innersurface of each substrate through the surface treating agent layer, theliquid crystal orienting film of at least one of the two substratesconstituted by a polymer, the polymer having a dipole moment of not lessthan 20 debye, the liquid crystal orienting films formed on the twosubstrates each having a dipole moment that is oriented in the samedirection with respect to the direction perpendicular to the substrates;and a liquid crystal sealed between the substrates, such that when avoltage is applied to the liquid crystal orienting film, alignment ofmolecules in the liquid crystal orienting film is disturbed.
 8. Thedevice according to claim 7, wherein the liquid crystal orienting filmsare uniaxially aligned.